1. Field:
This invention relates to the production of hydrides of Group IV-b and V-b transition metals and their alloys. It is specifically directed to the production of such hydrides from molten metal droplets.
2. State of the Art:
The preparation of metal hydrides, either intentionally or as an inherent consequence of other procedures is well known. The technology of effecting an absorption of hydrogen gas by a transition metal to form a hydride has been well developed in connection with the production of sponge or alloyed metallic materials predominating in metals of Group IV-b and V-b (notably Ti, Zr, Hf, V, Nb, Ta) of the periodic table of elements. The hydriding process of reactive metals as conventionally practiced is disclosed, for example, by U.S. Pat. Nos. 3,376,107; 3,776,855; and 4,470,847, the disclosures of which are incorporated herein by reference.
Metal hydrides, typically of the formula MH.sub.2, wherein M is a metal capable of reacting with hydrogen, are characteristically brittle. This characteristic is useful in the manufacture of particulate metals. For example, a sheet of metal may be heated to its reactive temperature and exposed to hydrogen gas. After a period of several hours, the metal becomes substantially hydrided; that is, the metal absorbs about one to three or more percent by weight (depending upon the molecular weight of the metal) hydrogen. It is thus brittle and susceptible to comminution to powdered form. The powder can then be heated or otherwise treated to remove the hydrogen. U.S. Pat. No. 4,300,946, the disclosure of which is incorporated by reference, discloses, for example, a method whereby metal is heated in the presence of hydrogen while concurrently being subjected to mechanical impact to obtain a particulate hydride of average particle size of less than about one centimeter.
Plasma generators have been used in various contexts in connection with the preparation or treatment of reactive metal hydrides. U.S. Pat. Nos. 3,803,403; 3,843,352; and 3,848,068, the disclosures of which are incorporated herein by reference, disclose representative procedures utilizing plasma heating. The process of U.S. Pat. No. 3,803,043 utilizes a plasma arc to generate spherical, but non-friable particles. The particles are then fabricated into an object, and the object is thereafter utilized as a hydrogen storage (through reversible hydriding) device. U.S. Pat. No. 3,843,352 utilizes a gas plasma in a cooled metal crucible to melt sponge metal. U.S. Pat. No. 3,848,068 vaporizes metal powder with a plasma and rapidly quenches the vapor to obtain ultra pure finely divided metal compounds, such as metal hydride. The plasma gas may contain up to 100 percent reactive gas; e.g., hydrogen. Particulate metal; e.g., zirconium and its alloys, preferably of minus 200 mesh size, is introduced to the plasma stream. The metallic material is vaporized, and the vapors react with the reactant gas of the plasma. The plasma stream and reaction products are rapidly quenched. Other processes have been suggested whereby a hydrogen gas plasma is directed against a pool of molten metal to supersaturate a region of the pool with hydrogen. The supersaturated composition migrates to a cooler region of the pool whereupon the absorbed hydrogen is expelled and carries microfine solid pure metal out of the pool for recovery.
The phase diagrams of the transition metals and their alloying elements are characterized by diverse phases occuring at the temperatures at which hydrogen absorption is promoted. (A representative such diagram for zirconium appears in U.S. Pat. No. 3,776,855.) Accordingly, there is a tendency for an alloy to degrade in composition during the hydriding procedure. Moreover, hydrogen solubility in the metals of most interest, notably zirconium, is lower at elevated temperatures, particularly in the molten metal range. Generally, to obtain adequate hydrogen absorption, it is necessary to expose the metal to hydrogen at the temperature range of about 400.degree. C. to about 800.degree. C. In the transition metal alloy systems, the various alloying metals tend to segragate into different phases if held too long in this temperature range. For example, in the zirconium system, iron, chromium and niobium tend to segragate into a beta phase, while tin segragates into an alpha phase.
There remains a need in the art for a method whereby hydrides of transition metal alloys may be formed rapidly without degradation.